The storage carbohydrates found in plants, including sucrose, glucans, starch and fructans, are an important source of feed for animals, particularly grazing ruminants. Often these carbohydrates can be of limited availability in pasture plants.
The nutritional value of forage-based diets for ruminants is limited by the microbial efficiency of the rumen. Rumen microorganisms require protein and carbohydrates to synthesize microbial protein and volatile fatty acids. Either the protein or carbohydrate can limit the microbial efficiency. If a diet has high levels of soluble protein, adequate quantities of readily fermentable carbohydrates should be included in the diet to avoid ammonia loss.
Excessive protein degradation in the rumen of animals may be the most limiting nutritional factor in legume or grass pastures or silage. Such pastures have the potential to produce a forage with high protein levels but this protein is not always utilized efficiently. Such pastures exhibits a high protein-low energy imbalance. Due to this energy imbalance, the ruminant fed with a high proportion of legume or grass forage, such as with grazing pure alfalfa, cannot fully benefit from the high protein content of the forage.
The cost of feeding a highly degradable protein source goes beyond the nitrogen losses and reduced microbial efficiency. There is an energy cost in detoxifying excess ammonia resulting from excessive rumen degradable protein. When comparing a 17% crude protein vs. a 19% diet, this maintenance cost is equal to one (1) pound of milk per cow daily (Shultz T. On-line, Milk Lines (July 1997).
hftp//www.ucce.tulare.ca.us/pub/milk0797.htm#MUN
In mammals the detoxification of ammonia is accomplished by the liver through the urea cycle. Carbamoyl phosphate is the starting point of the urea cycle and carbamoyl phosphate synthase uses energy from ATP to fuse a carbon dioxide molecule with an ammonia molecule and a phosphate to make an active form of ammonia that can be added to an acceptor molecule to make urea. (Makemson J. & Kuhn (Online). Amino acid catabolism and the urea cycle.
http://www.fiu.edu/.about.biology/bch3033/lectures/webureacyc.htm
This maintenance cost is perceived by some animal nutritionists as being the most important cause of reduced efficiency of highly degradable proteins in the feedstuff.
Legume digestibility declines during maturation of the plant as glucose and sucrose are remobilized in the plant and therefore cannot be stored in aerial parts effectively. However, the production of immobile carbohydrate sources such as glucans or fructans in aerial plant parts offer great potential to improve forage digestibility in legumes and grasses.
Good ensuing conditions for forages depend on the quick attainment and maintenance of an oxygen-free condition. The aerobic degradative processes are inhibited through the elimination of atmospheric oxygen, the formation of organic acids and a pH of 4 to 5. (Muck, 1988). The type of further ensuing activity or changes depends on the composition of the crop and the microorganisms present. Crops that have a naturally high level of carbohydrates ferment rapidly, produce a great deal of lactic acid, a low pH, and a generally high silage quality.
Many forages, such as legumes, however, do not produce good natural fermentation. When the level of carbohydrates is low, the amount of acid that is produced is low. Also, the high protein percentage in legumes tends to act as a buffer, as well as a source of ammonia and amine-type compounds, therefore making the attainment of a low pH even more difficult.
Compositional factors are near critical levels so that unfavorable fermentation (low palatability and high spoilage losses) frequently occur unless special precautions are taken. (Barnes and Gordon, 1972). The most popular additive to help the preservation of the forage involve the use of inoculants and the use of acids. Sugars such as molasses are also added to help the pH reduction process.
Extensive conversion of protein to non-protein nitrogen occurring during silage fermentation results in excessive production of ammonia in the rumen. Because milking cows fed high-protein alfalfa as their principal forage may receive as much as 60% of their total protein from the alfalfa, it is important to minimize protein degradation during harvest and storage. Degradation is greatest in the direct-cut silages but wilted alfalfa silages (haylage) may have as much as 20% ammonia N. (Conrad and Klopfenstein, 1988).
Barnes, R. F. & C. H. Gordon. 1972. Feeding value and on-farm feeding. pp. 601-630. In: Alfalfa Science and Technology. C. H. Hanson (ed.) No. 15 in the series Agronomy. ASA, Madison, Wis.
Bethard, G. (On-line) Estimating Rumen Available and Nonstructural Carbohydrates in Dairy Cattle Diets. Available hftp://www.cyber.vt.edu/dl/cows/9718.html.
Conrad H. R. & T. J. Klopfenstein. 1988. Role in livestock feeding-greenchop, silage, hay, and Dehy. pp. 539-551 in: Alfalfa and Alfalfa Improvement; C. H. Hanson (ed.). No 29 in the series Agronomy.
Franck, R. 1995. The Balancing Act. Dairy Herd Management, February 1995: 50-52.
Muck, R. E. 1988. Factors Influencing Silage Quality and their Implications for Management. J. Dairy. Sci. 71: 2992-3002.
Nocek, J. E., and J. B. Russell. 1988. Protein and energy as an integrated system. Relationship of ruminal protein and carbohydrate availability to microbial synthesis and milk production. J. Dairy Sci. 71:2070.
Paterson, J. A.; R. L. Belyea, J. P. Bowman, M. S. Kerley & J. E. Williams. 1994. The impact of forage quality and supplementation regimen on ruminant animal intake and performance. pp. 59-114 in: Fahey, G. C. Jr. (ed.) Forage Quality, Evaluation, and Utilization. ASA, CSSA, SSSA, Madison, Wis.
Stokes, S. R., W. H. Hoover, T. K. Miller & R. Blauweikel. 1991. Ruminal digestion and microbial utilization of diets varying in types of carbohydrate and protein. J. Dairy Sci. 74: 871-881.
Vagnoni, D. B. & G. A. Broderick. 1995. Effect of Energy supplementation of alfalfa hay or alfalfa silage on protein supply to lactating cows. U.S. Dairy Forage Research Center. 1995 Research Summaries.
Van Keuren, R. W. and A. G. Matches. 1988. Pasture production and utilization. pp. 515-551 in: Alfalfa and Alfalfa Improvement; C. H. Hanson (ed.). No 29 in the series Agronomy.